The invention relates to a self-counter sinking screw. Different kinds of self counter-sinking screw are know from prior art.
It is known, e.g. from U.S. Pat. No. 5,203,657 (Nagoshi et al.) to provide a self-tapping screw, where the generally conical undersurface of the head of the screw is particularly configured to prevent the bulging up of a gypsum board cover sheet. In one embodiment, the underside of the head is provided with a series of equidistantly spaced apart spot facing projection which also provide the burring action to make space for the inversely conical underside of the head. This arrangement may be useful with gypsum board panels or the like material, where the material of the board is soft, has a relatively small density and is easily displaceable or compressible. It would be inoperative when the screw would have to be used with hard finish panels such as melamine, where the material cut from the panel cannot be removed toward the threaded portion of the screw.
U.S. Pat. No. 5,199,839 (DeHaitre) discloses a self locking screw provided at its underside with a number of flat self locking projections having an arcuate edge. The edge may be used in burring action to provide a part of the seat for the underside of the screw head. However, since there is no provision for effective disposal of the burrs, the arrangement is only suitable in applications where the workpiece is from material such as wood composite material, but not with a hard, dense material.
U.S. Pat. No. 3,903,784 (Dekker) shows another self-countersinking fastener head, where the cutter edges at the underside of the head are arcuately curved. Each cutter edge defines a trailing end of a cutter recess. The volume of the recess decreases in the direction from the threaded stem to the top of the head. This is disadvantageous from the standpoint, firstly, of the manufacturing costs as a curved edge is relatively difficult to produce. Secondly, the upwards diminishing volume of the cutter recess does not provide sufficient space for the burrs from immediately adjacent areas of the workpiece as most of the material in self-countersinking is removed at the upper, larger diameter portion of the countersink.
U.S. Pat. No. 465,101 (Richards) shows an arrangement wherein knives or blades are forged to project from the truncated cone of the underside of the screw head. The edges of the knives project beyond the contour of the truncated cone. Therefore, they take away more material than is required for countersinking the seat for the underside of the screw, which weakens the strength of the engagement between the head and the workpiece.